The present invention relates to an optical compensatory sheet comprising a transparent substrate, an orientation layer and an optically anisotropic layer comprising discotic liquid crystal molecules in order. The invention also relates to a liquid crystal display of an STN mode. The invention further relates to a method of alignment of discotic liquid crystal molecules at an average inclined angle in the range of 50xc2x0 to 90xc2x0.
A liquid crystal display of an STN mode comprises a liquid crystal cell of an STN (Super Twisted Nematic) mode, two polarizing elements arranged on each side of the liquid crystal cell, and one or two optical compensatory sheets (phase retarders) arranged between the liquid crystal cell and the polarizing element.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal molecule. Each of the substrates has an orientation layer, which has a function of aligning the rod-like liquid crystal molecule. The rod-like liquid crystal molecules are twisted by using a chiral agent. A twist angle of the molecules is in the range of 180xc2x0 to 360xc2x0.
Without use of an optical compensatory sheet, a liquid crystal display of an STN mode displays an image colored blue or yellow because of birefringence of rod-like liquid crystal molecules. The blue or yellow color is inconvenient for not only a black and white image but also a color image. An optical compensatory sheet has a function of removing color from an image to display a bright and clear image. The optical compensatory sheet also has a function of enlarging a viewing angle of a liquid crystal cell. A stretched birefringent film has usually been used as the optical compensatory sheet. Japanese Patent Provisional Publication Nos. 7(1995)-104284 and 7(1995)-13021 disclose an optical compensatory sheet made of a stretched birefringent film for a liquid crystal display of an STN mode.
An optical compensatory sheet comprising an optically anisotropic layer on a transparent substrate has been proposed to be used in place of the stretched birefringent film. The optically anisotropic layer is formed by aligning discotic liquid crystal molecules and fixing the aligned molecules. The discotic liquid crystal molecules usually have large birefringence. The discotic liquid crystal molecules also have various alignment forms. Accordingly, an optical compensatory sheet obtained by using the discotic liquid crystal molecule has a specific optical characteristic that cannot be obtained by the conventional stretched birefringent film. The optical compensatory sheet using the discotic liquid crystal molecule is disclosed in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620A1. However, the disclosed optical compensatory sheet is designed to be used in a liquid crystal display of a TN (Twisted Nematic) mode.
The optical compensatory sheet containing the discotic liquid crystal molecules should be redesigned to be used in a liquid crystal display of an STN.mode. In a liquid crystal cell of the STN mode, rod-like liquid crystal molecules are aligned according to a super twisted birefringent mode at a twist angle of larger than 90xc2x0. The liquid crystal display of the STN mode can display a clear image of a large volume according to a time-sharing addressing method even though the display has a simple matrix electrode structure (having no active matrix such as a thin film transistor or a diode).
The discotic liquid crystal molecules should be essentially vertically aligned (homogeneously aligned) to optically compensate the liquid crystal cell of the STN mode. The discotic liquid crystal molecules are preferably further twisted. Japanese Patent Provisional Publication No. 9(1997)-26572 discloses an optical compensatory sheet in which discotic liquid crystal molecules are twisted. The drawings of Japanese Patent Provisional Publication No. 9(1997)-26572 further illustrate that discotic liquid crystal molecules are essentially vertically aligned.
It is technically difficult to align discotic liquid crystal molecules uniformly (monodomain alignment) from an interface facing an orientation layer to another interface facing the air according to the disclosures of Japanese Patent Provisional Publication No. 9(1997)-26572. If the discotic liquid crystal molecules are not uniformly aligned, disclination of the molecules causes scattered light, which decreases a contrast ratio of a displayed image.
Rod-like liquid crystal molecules used in a liquid crystal cell have been investigated to align the molecule essentially vertically (homeotropic alignment). For example, a liquid crystal display of a vertical alignment (VA) mode uses an orientation layer having a function of essentially vertically aligning rod-like liquid crystal molecules. In the liquid crystal display of the VA mode, rod-like liquid crystal molecules are essentially vertically aligned while not applying voltage to the cell, and are essentially horizontally aligned while applying voltage to the cell. Various orientation layers have been proposed to align rod-like liquid crystal molecules.
Discotic liquid crystal molecules are completely different from the rod-like liquid crystal molecules in molecular structures and in optical characteristics. Most of the orientation layers having a function of aligning rod-like liquid crystal molecules are not effective in aligning discotic liquid crystal molecules.
An object of the present invention is to provide an optical compensatory sheet suitable for a liquid crystal display of an STN mode.
Another object of the invention is to provide a liquid crystal display of an STN mode that can display a clear image of a high contrast, in which blue or yellow color caused by birefringence of rod-like liquid crystal molecule is reduced.
A further object of the invention is to provide a method for vertical, uniform and stable alignment of discotic liquid crystal molecules.
The present invention provides an optical compensatory sheet comprising a transparent substrate, an orientation layer and an optically anisotropic layer in order, said optically anisotropic layer comprising discotic liquid crystal molecules, wherein the orientation layer comprises a polymer having a hydrocarbon side chain containing 10 or more carbon atoms, said discotic liquid crystal molecules being aligned at an average inclined angle in the range of 50xc2x0 to 90xc2x0.
The invention also provides a liquid crystal display comprising a liquid crystal cell of an STN mode, two polarizing elements arranged on each side of the liquid crystal cell and one or two optical compensatory sheets arranged between the liquid crystal cell and the polarizing elements, wherein the optical compensatory sheet comprises a transparent substrate, an orientation layer and an optically anisotropic layer in order, said transparent substrate being adjacent to the polarizing element, said optically anisotropic layer comprising discotic liquid crystal molecules, said orientation layer comprising a polymer having a hydrocarbon side chain containing 10 or more carbon atoms, said discotic liquid crystal molecules being aligned at an average inclined angle in the range of 50xc2x0 to 90xc2x0.
The invention further provides a method of alignment of discotic liquid crystal molecules, which comprises forming an optically anisotropic layer comprising discotic liquid crystal molecules on an orientation layer comprising a polymer having a hydrocarbon side chain containing 10 or more carbon atoms to align the discotic liquid crystal molecules at an average inclined angle in the range of 50xc2x0 to 90xc2x0.
The invention furthermore provides an optical compensatory sheet comprising a transparent substrate, an orientation layer and an optically anisotropic layer in order, said optically anisotropic layer comprising discotic liquid crystal molecules, wherein the orientation layer contains a fluoropolymer, said discotic liquid crystal molecules being aligned at an average inclined angle in the range of 50xc2x0 to 90xc2x0.
The invention still further provides a liquid crystal display comprising a liquid crystal cell of an STN mode, two polarizing elements arranged on each side of the liquid crystal cell and one or two optical compensatory sheets arranged between the liquid crystal cell and the polarizing elements, wherein the optical compensatory sheet comprises a transparent substrate, an orientation layer and an optically anisotropic layer in order, said transparent substrate being adjacent to the polarizing element, said optically anisotropic layer comprising discotic liquid crystal molecules, said orientation layer comprising a fluoropolymer, and said discotic liquid crystal molecules being aligned at an average inclined angle in the range of 50xc2x0 to 90xc2x0.
The invention still furthermore provides a method of alignment of discotic liquid crystal molecules, which comprises forming an optically anisotropic layer comprising discotic liquid crystal molecules on an orientation layer comprising a fluoropolymer to align the discotic liquid crystal molecules at an average inclined angle in the range of 50xc2x0 to 90xc2x0.
In the present specification, the term xe2x80x9caverage inclined anglexe2x80x9d means an average of angles between discotic planes of discotic liquid crystal molecules and a surface of a transparent substrate (or a surface of an orientation layer). The present specification refers to alignment of discotic liquid crystal molecules at an average inclined angle in the range of 50xc2x0 to 90xc2x0 as essentially vertical alignment of the molecules.
The applicants have succeeded in obtaining essentially vertical, uniform and stable alignment of discotic liquid crystal molecules by using a polymer having a hydrocarbon group containing 10 or more carbon atoms in its side chain or a fluoropolymer in an orientation layer.
An optical compensatory sheet suitable for a liquid crystal display of an STN mode is now obtained by using the discotic liquid crystal molecules of the essentially vertical, uniform and stable alignment. Thus, blue or yellow color is reduced in a liquid crystal display of an STN mode to display a clear image of a high contrast by using an optical compensatory sheet, in which the discotic liquid crystal molecules are essentially vertically aligned (and preferably twisted).